1. Field of the Invention
The present invention relates to a scanning probe microscope capable of performing accurate profile measurement of samples including steep sloped surfaces and soft materials.
2. Description of the Related Art
Scanning probe microscopes (SPM) have been known for measuring microscopic three-dimensional profiles. The art of scanning probe microscopes relates to controlling a probe having a pointed tip to scan samples while maintaining an extremely small contact force, which is used extensively for measuring atom-order microscopic three-dimensional profiles.
On the other hand, dimensional control using CD-SEM (length measurement SEM) is performed currently in the process of forming microscopic patterns on an LSI, but as the patterns are being scaled-down in size, the following problems of limitations have become evident. The first problem relates to measurement accuracy. The gate width of a 45-nm node LSI, which is considered to be the mainstream in the future, is 25 nm, and the required measurement accuracy thereof is 0.5 nm, assuming that the permissible variation is 10% and the measurement accuracy is 20% thereof. The second problem relates to the demand of profile measurement. The necessity of APC (advanced process control) for highly accurate control of line width has increased, which requires a measurement method for measuring not only the pattern line width but also the cross-sectional profile that influences the electrical characteristics greatly. The third problem relates to the measurement object. There are increasing needs for measuring materials having little tolerance to electron beams, such as a DUV (deep ultra violet) resist or a low-k (low-dielectric constant) film material. Further, similar measurement needs such as measurement accuracy, need for profile measurement and need for resist pattern measurement for creating masters exist in the measurement of pits of next-generation high-density optical disk memories.
The art of scanning probe microscopes is considered promising in solving the above-described problems. However, since semiconductor patterns have an extremely high aspect ratio, there are drawbacks in that the measured profile data are deformed since the probe cannot follow the vertical movement of a steep sloped surface or since the probe slides on the steep sloped surface during probe scanning. Further, there are drawbacks in that the object to be measured is deformed by the contact force, and the quantity of deformation differs between a soft material and a hard material, so that error occurs in the measured profile when a sample including different surface materials is measured.
With respect to these problems, Japanese patent application laid-open publication No. 2001-33373 (patent document 1) and No. 2004-132823 (patent document 2) discloses scanning methods of approximating the probe to the sample at discrete sampling positions and measuring the height at these positions when the contact force becomes constant, and then retracting the probe to move the probe to the next measurement point where the same measurement process is repeatedly performed. The disclosed methods overcome the problem of error caused by the probe not capable of following the vertical movement on a steep sloped surface, since the probe is not dragged by these methods. However, since the probe is driven toward the sample until the contact force reaches a constant value, the minute contact force still causes slight deformation of the sample or sliding of the probe, which causes error in the measured profile.
As described above, the prior art methods had drawbacks such as deteriorated measurement accuracy caused by the probe sliding on steep sloped surfaces on a sample having a high aspect ratio or by deformation of the sample including soft materials.